A veteran Republican activist whose quest to obtain Hillary Clinton’s emails from hackers dominated the final months of his life struck up a professional relationship with Lt. Gen. Michael Flynn, the former national security adviser to President Trump, as early as 2015, and told associates during the presidential campaign that he was using the retired general’s connections to help him on the email project.

The late Peter W. Smith, an Illinois financier with a long history in Republican politics, met with Mr. Flynn in 2015, according to people familiar with the matter. At the time, Mr. Flynn had recently left his job as head of the Defense Intelligence Agency and was trying to set up his own consulting firm, while Mr. Smith was looking at investment opportunities in cybersecurity.

Mr. Flynn was a top adviser to Mr. Trump during the 2016 campaign. He served briefly as national security adviser in Mr. Trump’s administration before being forced to resign and pleading guilty to a charge of lying to authorities about his conversations with a Russian ambassador. He is scheduled to be sentenced in December.

Additionally, in an email reviewed by The Wall Street Journal, one of Mr. Smith’s former associates wrote to a friend last week, “As you are aware Peter started a business relationship with Gen. Mike Flynn in November 2015. We spoke with him on the day he left for his trip to Moscow.” The associate, John Szobocsan, sent the email as the Journal was preparing a story on Mr. Smith and was attempting to reach Mr. Szobocsan. He didn’t respond to requests for comment.

The Journal reported in 2017 that Mr. Smith implied he had connections to Mr. Flynn, but the email and people familiar with the matter indicate the two men were in contact and did in fact have a working relationship. Though no apparent business deals came of the 2015 meeting, the introduction gave Mr. Smith a contact who would go on to become part of Mr. Trump’s inner circle. An attorney for Mr. Flynn declined to comment.

Mr. Smith’s pursuit of Mrs. Clinton’s emails put him in contact with a varied group of hackers and operatives on the fringes of Republican politics and brought him to dark corners of the online world, the Journal has previously reported. He died in a Minnesota hotel room in 2017 in what authorities ruled a suicide at the age of 81, weeks after telling friends that he believed he had finally obtained the missing emails, according to a person familiar with the matter. His death came 10 days after describing his efforts to a reporter for the Journal.

How much of Mr. Smith’s quest was undertaken with the knowledge of anyone in Mr. Trump’s orbit is a question investigators have been probing for more than a year.

The matter is under scrutiny by Special Counsel Robert Mueller, who is investigating Russian interference in the 2016 election and any possible collusion between the Trump campaign and Russia. Numerous people familiar with Mr. Smith’s quest have been questioned by Mr. Mueller’s investigators, including at least one witness who was called before a grand jury, according to a court document and people familiar with the matter. A grand jury subpoena described to the Journal also has sought a range of documents concerning Mr. Smith’s activities. President Trump has denied any collusion. The Russian government has denied it interfered in the election.

People who knew Mr. Smith, including former Wall Street financier Charles Ortel and freelance writer Tom Lipscomb, describe his quest as all-consuming, and say he believed Mrs. Clinton’s emails would reveal a vast amount of incriminating information. Mrs. Clinton described the 33,000 emails that her lawyers deleted from her personal server as relating to routine matters such as yoga.

Mr. Smith formed a company called KLS Research as a vehicle for his project, the Journal previously reported. He reached out to businessmen as financial backers, including Maine real-estate developer Michael Liberty, Florida-based investor John “Jack” Purcell and Chicago financier Patrick Haynes. They were named in an email reviewed by the Journal as among a group of people who pledged to contribute $100,000 to the effort, along with $50,000 of Mr. Smith’s own money.

Messrs. Haynes and Purcell didn’t respond to requests for comment. An attorney for Mr. Liberty said his client never donated but may have been asked. The Journal couldn’t determine the identity of a fourth name in the email.

Mr. Lipscomb also told the Journal he believed Mr. Smith was soliciting money from friends, including himself, under false pretenses. In an interview, he said he now thinks Mr. Smith was a “deluded old man” who had “bet the farm” on misguided efforts to find the emails.

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Beyond his connection with Mr. Flynn, Mr. Smith also claimed ties with the antisecrecy website WikiLeaks, and he solicited money to assist founder Julian Assange with legal support, according to Mr. Ortel and an email Mr. Smith sent in December 2016 that was reviewed by the Journal.

In the email, an update on what Mr. Smith called the “Clinton Email Reconnaissance Initiative,” he told supporters his team had come across “multiple individuals” in possession of the Clinton emails in the fall of 2016. He also wrote that he directed one or more of those people to send the emails to WikiLeaks.

WikiLeaks, which never published any of the alleged material, was the main conduit for the dissemination of other embarrassing Democratic emails that U.S. investigators concluded were stolen by Russian hackers. No evidence has emerged that hackers ever obtained Mrs. Clinton’s emails, and the FBI didn’t find evidence that her personal server had been compromised.

“We have active discussions on how to arrange such legal support for WikiLeaks as a whistleblower, and therefore being exempt from U.S. law,” Mr. Smith wrote.

WikiLeaks didn’t respond to a request for comment. Mr. Assange, who has been at the Ecuadorian embassy in the United Kingdom for over six years, has repeatedly denied that the group obtained any hacked Democratic material from Russian sources.